TF-616 Add adr for handle heavy task with isolate

docs/adr/0008-handle-heavy-task-with-isolate.md

@@ -0,0 +1,48 @@
+# 8. Handle heavy task with isolate
+
+Date: 2022-06-29
+
+## Status
+
+Accepted
+
+## Context
+
+- Use isolate to handle heavy task
+
+## Decision
+
+- Initialize the IsolateManager to help manage the list of tasks and isolate
+```
+final isolateManager = IsolateManager();
+await isolateManager.initialize(isolatesCount: 1);
+```
+ + `isolatesCount`: The number of isolates created and run concurrently. Depends on your device's processor.
+
+- Perform task processing by calling `execute` function
+```
+isolateManager.execute(
+    arguments,
+    handleTaskAction,
+    onUpdateProgress
+);
+```
+ + `arguments`: Parameters passed to the function handling heavy tasks.
+ + `handleTaskAction`: Function to handle heavy tasks
+ + `onUpdateProgress`: Callback function every time a value is sent from `new isolate` to `main isolate` via `sendPort`
+
+- The return value is a `Completer`
+
+- Communicate between two isolates
+To see a visual representation of this process, take a look at the image below:
+
+[communicate_between_two_isolate](../images/handle_task_with_isolate.png);
+
+## Consequences
+
+- No UI block when handling long-term tasks.
+- Increase application performance, smoother application.
+
+## Reference
+
+- [worker_manager](https://pub.dev/packages/worker_manager)